This invention relates in general to polymers, and in particular to ionically conductive polymers for use in fuel cells and other electrochemical devices. For example, the polymers may be used as binders in fuel cell electrodes and/or for producing fuel cell membranes.
A fuel cell is an electrochemical device that continuously converts chemical energy into electric energy and some heat for as long as fuel and oxidant are supplied. Fuel cells are evolving. Some currently known categories of fuel cells include polymer electrolyte membrane (PEM), alkaline, phosphoric acid, molten carbonate, solid oxide, and microbial/enzyme based.
At the heart of the PEM fuel cell is a membrane electrode assembly (MEA). The MEA includes a membrane made from a polymer electrolyte or proton conducting polymer. The polymer electrolyte membrane is sandwiched between a pair of electrodes called an anode and a cathode. The MEA also usually includes porous, electrically conductive sheets called gas diffusion layers positioned adjacent to the electrodes to permit diffusion of reactants to the electrodes.
In operation, a fuel such as hydrogen or methanol is flowed into contact with the anode where it dissociates into electrons and protons. The electrons, which cannot pass through the membrane, flow from the anode to the cathode through an external circuit containing an electric load, which consumes the power generated by the cell. The protons pass through the membrane and combine with oxygen and electrons on the cathode to produce water and heat.
The electrodes are applied as thin layers on opposing sides of the membrane. The electrodes include particles of an electrocatalyst such as platinum. The catalyst particles are often supported on electron conducting carrier particles such as carbon particles. The electrodes are typically formulated by combining the catalyst and carrier particles with an ionically conductive polymer, often referred to as a binder, that holds the particles together and provides the electrode with mechanical integrity. The ionically conductive polymer is proton conducting and is sometimes electron insulating and other times electron conducting depending on the particular polymer(s). A fluorinated polymer sold by DuPont under the tradename Nafion™ is often used as the binder.
U.S. Patent Application No. 2006/0036064 A1 by McGrath et al., published Feb. 16, 2006, discloses sulfonated polymers for use as binders in fuel cell electrodes. Specific examples of polymers include polysulfones, polyimides, polyketones, and poly(arylene ether phosphine oxide)s. The sulfonated polymer can be used to form the polymer electrolyte membrane as well as the anode and/or cathode of a membrane electrode assembly.
U.S. Pat. No. 6,964,823 B2 by Koyama et al., issued Nov. 15, 2005, discloses polymers such as poly-ether ether ketone and poly-ether sulfone having sulfonated side chains. The polymers can be used in a membrane electrode assembly of a fuel cell both to form the polymer electrolyte membrane and as a binder in the electrodes.
Von Kraemer et al., “Gas diffusion electrodes and membrane electrode assemblies based on a sulfonated polysulfone . . . ”, Journal of the Electrochemical Society 153 (11), 2077-2084 (2006), discloses a membrane electrode assembly in which a sulfonated polysulfone is used as a binder in the electrodes and is used to make the polymer electrolyte membrane.